A simulation of active transport in an enzyme-immobilized bipolar membrane

In this study the transport phenomena across an enzyme-immobilized bipolar membrane was simulated, where the enzyme was fixed between positively and negatively charged layers. The transport phenomena of the products was simulated with Donnan's equilibrium and Nernst-Planck's equation of ion flux under the assumption that the hydrolytic reaction formula of urea caused by urease was represented by the following equation: (NH2)(2)CO + 2H(2)O --> 2NH(4)(+) + CO2 - 3 The calculated results predicted that all products were mainly transported to the cell face to the positively charged layer, which implies an active transport in the model system. The magnitude of such a directional transport is a function of the physicochemical parameters in the membrane system, such as membrane thickness, charge density, ion mobility, substrate concentration and the concentration of the products in the membrane. The experimental results for the carbonate ion correspond well with the theoretical :prediction. On the other hand, those for the ammonium ion deviate from it. The membrane potential change generated between both sides of this membrane was calculated as a function of time. The theoretical result predicts that it has a maximum peak at the initial stage of this system. When the membrane thickness is very thin, it becomes very sharp like a pulse. The experimental results support the theoretical prediction even though it was a broad peak because the measurement was performed with a thick membrane. (C) 1997 Elsevier Science B.V.